support transparent render background

It is useful for export.

src/dialog/vexportdialog.cpp

@@ -370,6 +370,7 @@ void VExportDialog::initUIFields(MarkdownConverterType p_renderer)
 
     // Markdown rendering background.
     m_renderBgCB->addItem(tr("System"), "System");
+    m_renderBgCB->addItem(tr("Transparent"), "Transparent");
     const QVector<VColor> &bgColors = g_config->getCustomColors();
     for (int i = 0; i < bgColors.size(); ++i) {
         m_renderBgCB->addItem(bgColors[i].m_name, bgColors[i].m_name);

src/vconfigmanager.cpp

@@ -1487,7 +1487,9 @@ void VConfigManager::clearGroupOfSettings(QSettings *p_settings, const QString &
 
 QString VConfigManager::getRenderBackgroundColor(const QString &p_bgName) const
 {
-    if (p_bgName != "System") {
+    if (p_bgName == "Transparent") {
+        return "transparent";
+    } else if (p_bgName != "System") {
         for (int i = 0; i < m_customColors.size(); ++i) {
             if (m_customColors[i].m_name == p_bgName) {
                 return m_customColors[i].m_color;

src/vmainwindow.cpp

@@ -1528,6 +1528,17 @@ void VMainWindow::initRenderBackgroundMenu(QMenu *menu)
     if (curBgColor == "System") {
         tmpAct->setChecked(true);
     }
+
+    renderBgMenu->addAction(tmpAct);
+
+    tmpAct = new QAction(tr("Transparent"), renderBackgroundAct);
+    tmpAct->setToolTip(tr("Use a transparent background for Markdown rendering"));
+    tmpAct->setCheckable(true);
+    tmpAct->setData("Transparent");
+    if (curBgColor == "Transparent") {
+        tmpAct->setChecked(true);
+    }
+
     renderBgMenu->addAction(tmpAct);
 
     const QVector<VColor> &bgColors = g_config->getCustomColors();
@@ -1810,6 +1821,7 @@ void VMainWindow::setRenderBackgroundColor(QAction *action)
     if (!action) {
         return;
     }
+
     g_config->setCurRenderBackgroundColor(action->data().toString());
     vnote->updateTemplate();
 }